Pressure sensitive adhesives, commonly known as PSA, are self-adhesive materials designed to stick on a surface by simple contact under light pressure. The application of pressure is necessary in order to achieve sufficient wet-out onto the substrate surface to provide adequate adhesion. The mode of bonding for a PSA to the substrate is not chemical, in fact this class of adhesives does not undergo any chemical reaction during the bonding process.
Pressure sensitive adhesive compositions are formulated to provide compositions having a certain minimum balance of physical and chemical properties so as to be able to withstand the stress and environment that the product will encounter and be expected to survive in use. The physical or rheological properties of the adhesive, particularly tack, peel adhesion and shear adhesion, must be finely tuned for the application, combining a careful chosen polymer architecture with the proper addition of small molecules such as tackifying resins.
PSAs are particularly easy and safe to use since no chemical reaction takes place and bonding can be done at room temperature.
Pressure sensitive adhesives are based on polymers mainly coming from acrylics, styrenic block copolymers, natural rubber and polyurethanes. There are also niche markets for silicone PSAs, where low temperature use or high-temperature stability is required and cost is not an issue. Rubber-based adhesives are cheap to produce and also the simplest to formulate, since they are typically compounds of natural rubber and low molecular weight tackifying resin miscible with the rubber in similar proportions.
Earlier versions were not cross-linked, but today a cross-linking step is generally performed to avoid flow.
Acrylate copolymer based PSAs are available in a variety of forms (solutions, emulsions, and even hot melts) and have a wide range of properties. However, these adhesives require to be cured after application through the use of curing agents or require the use of energy to dry or cure them after application. Thus, as for natural rubber PSAs, a cross-linking step (electron-beam or UV radiation), once the adhesive has been coated, is generally used to prevent creep.
PSAs based on styrenic block copolymers are generally blends of styrene-isoprene-styrene (SIS) triblocks and styrene-isoprene diblocks compounded with a low molecular weight resin based on C5 rings that is miscible with the isoprene phase but immiscible with the styrene phase. Thus, a tackifying resin is a necessary component for this class of PSA. The styrene domains provide physical cross-links which give rise to superior resistant to creep.
Other formulations of PSA are based on polyurethane pre-polymers having NCO groups available for reaction with water or other chain extenders. Said pre-polymers result from the reaction of polyols, usually obtained by addition polymerization of ethylene oxide or propylene oxide with a polyisocyanate. As an example, EP1983012, EP0196749 and U.S. Pat. No. 3,681,277 describe the preparation of urethane resins or pre-polymers of polyurethane for the manufacture of PSA by reaction of polyisocyanates with polyols derived from polyoxyalkylene compounds.
As an alternative to these materials, polyester or aliphatic polycarbonate polyol compounds have also been used to produce PSA formulations. WO98/38262 describes a pressure sensitive adhesive layer comprising a polyester component and one or more epoxy resins, which is cross-linkable upon exposure to actinic or e-beam irradiation.
US2002/081426 discloses a PSA sheet or layer whose main component is an aliphatic polycarbonate diol of formula —(O—R—O—C(O))1— wherein R is a linear or branched hydrocarbon group.
Removable adhesives constitute one particular class of PSAs. These adhesives are designed to form a temporary bond, and ideally can be easily removed after long periods of time without leaving any residue on the surface to which they are adhered.
Removable adhesives are used in applications such as surface protection films, masking tapes, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact. In this sense, Post it® is a particular example of a widely known and used removable adhesive in the prior art.
Some of these removable adhesives are designed to repeatedly stick and unstick, being also used, for example, in resealable packages, such as in food packaging industry, where the user can reseal the package after use, thus preserving freshness and allowing easy access to the product. Some of these formulations include ethylene-vinyl acetate copolymers or mixtures with styrene block copolymers (U.S. 2009/0270540; U.S. Pat. No. 3,330,670).
However, in these applications most of the adhesive formulations require the addition of high amounts of tackifying resin in order to improve the degree of stickiness and, therefore, the adhesion properties of the formulation, components which are in most cases harmful and not environmentally friendly.
Polyalkylene carbonate compounds have been used in the preparation of other formulations in combination with polyols. Particularly, mixtures of polypropylene carbonate and polyols, such as polyether polyols, polyester polyols and polycarbonate polyols have been described in the literature (EP2845878) for the manufacture of resin compositions which are cured in the presence of a curing agent, such as a polyisocyanate, to obtain an interpenetrating polymer network with improved physical properties. Similar mixtures have been described for the manufacture of adhesive formulations (WO2013/158621).
Polyether carbonate polyols have also been combined with polyols in the presence of urethane groups, either in the polyether carbonate polyol structure or in the polyol chain, to produce adhesive formulations and polyurethane foam materials (US2013/150526, US2014/066535 and US2013/059973).
However, mixtures of polyalkylene carbonate with polyether carbonate polyols have not been disclosed in the prior art. In order for these mixtures to be useful as a pressure sensitive adhesive, a suitable miscibility is required between the components constituting the mixture. Furthermore, good adhesion properties, particularly tack, peel adhesion and shear adhesion, are also required.
On the other hand, self-healing polymers (those having the ability to repair themselves autonomously) or healable polymers (those which can be healed upon exposure to an external stimulus, such as heat, light, pressure or mechanical stress) have been developed to make easier the repair of mechanical failures and crack formation in the polymer.
Particularly, a self-healing polymer must possess the ability to form multiple bonding interactions in and around the damaged area, creating connections between the components that make up its structure. This challenge has been treated in the prior art with four different strategies: encapsulation of reactive monomers that are released after a fracture; the formation of new irreversible covalent bonds in the damaged area; supramolecular self-assembly; and the formation of reversible covalent bonds. However, in spite of the various self-healing materials described in the prior art, there is still a need to provide with polymer materials having self-healing properties.